#1 (PET or PETE) plastic is the kind used for bottled water bottles, which are generally regarded as safe. They have been shown to leach antimony into the water in a couple of studies, but at levels considered safe by the EPA. The scare about their leaching DEHA if reused, which you may have come across, turns out to be an urban legend. If there is any risk from reuse, it probably comes from bacterial contamination. (The bottles' narrow necks make them hard to clean.)
#2 (HDPE), 4 (LDPE) and 5 (polypropylene) plastics are generally regarded as safe.
#3 (PVC) and 6 (styrene) plastics pose health risks and should be avoided. (They are not ordinarily used for water bottles, but are used for other food and beverage containers.)
#7 plastic is usually polycarbonate and contains BPA. If you are in love with a particular #7 bottle, you could call the manufacturer to identify the plastic, but that might not make matters clearer. The "better safe than sorry" approach would be to avoid #7 altogether in my opinion.